In exhaust gas discharged from combustion equipment such as a coal or heavy oil fired boiler, hazardous trace substances such as mercury exist, so that it is necessary to remove these hazardous substances from exhaust gas. It is thought that the mercury in exhaust gas generally is present as water-insoluble metallic mercury (Hg) and water-soluble mercury chloride (HgCl2). Although mercury chloride, which is easily absorbed by water, can be removed by a desulfurization absorption tower or the like, metallic mercury, which has a very low solubility in water, is not absorbed by a desulfurization absorption tower, and may be discharged as metallic mercury vapor without being absorbed.
Conventionally, therefore, there has been proposed, in addition to an activated carbon adsorption method, a sodium hypochlorite absorption method, etc.; a method in which metallic mercury is converted to mercury chloride on a catalyst by adding a chlorinating agent, for example, in a denitrification apparatus filled with the catalyst. The reason for this is that if metallic mercury can be converted to mercury chloride in the presence of a catalyst such as a denitrification catalyst, mercury can be removed by a downstream desulfurization apparatus (see, for example, Japanese Patent Provisional Publication No. 10-230137 (230137/1998)).
Also, since the denitrification catalyst is placed at a high temperature not lower than 300° C. in an exhaust gas treatment system, there has been proposed a method in which considering an adverse influence of the addition of chlorinating agent on the system, a mercury oxidation catalyst is installed separate from the denitrification catalyst (see, for example, Japanese Patent Provisional Publication No. 2001-242596). With this method, metallic mercury can be converted to mercury chloride by chlorine content in exhaust gas in the presence of the mercury oxidation catalyst without the addition of chlorinating agent, and thus mercury can be removed by the desulfurization apparatus.
However, both of the above-described methods have a problem in that when the mercury having been oxidized once is trapped by a downstream scrubber such as a cooling tower and the desulfurization apparatus, it is reduced by the action of SO2 coexisting in flue gas, and is volatized again.